Many stringed instruments, such as guitars (both electric and acoustic), use frets on a fingerboard that are separated at specific distances by fret wires. The distance between fret wires (i.e., the length of a fret or the fret spacing) is determined by the scale length of the guitar using a formula sometimes referred to as the rule of 18 or, more accurately, the rule of 17.817154. Because many guitars typically use scale lengths between 24.75 inches and 25.5 inches, the length of each fret on a guitar typically falls within a well-defined range.
Typical musicians must practice for many hours before achieving proficiency. In the case or guitars and other stringed instruments, musicians practice in order to train their hands to the size of the frets and the nature of the strings in order to improve their talents. At the same time, full size instruments are too large for easy portability, which limits when and where a musician can initiate a practice session.